agof: in most of the old games there's a surefire strategy that devs created to win, and the point of the game is to find it like in a puzzle. instead of trying to play it properly like in the modern games.
That hasn't been my experience for any of the conquest type campaign maps. Sure, some maps are gimmick maps or have a particular objective that you need to focus on (I'm thinking of HoMM3, where there is a particular Necromancer campaign map where your win condition is getting a particular hero across the map to a particular castle), but most of the HoMM1 campaign are standard matches.
Skill at the fundamentals is a huge part of what is needed, including a feel for what fights will turn out well and which fights won't (you can also use save scumming if you aren't good at this, reloading if the fight goes poorly). You need an understanding of what each unit does, which of your units need to be protected, and which units can be sacrificed to do that protecting.
I played through HoMM1 with the Knight faction first and did fine. The cheaper, more plentiful units work well. The last map gave me some trouble because all 3 AI are allied against you, but the standard strategy of "Move Fast, Hit Hard" still worked. I also played through the campaign as the Warlock; the stronger units are nice in combat, though the buildings are quite expensive.
There isn't a magic bullet way to win most of those maps. If you find yourself in a lost position on a campaign map, then take a look around at what you've explored (you've explored a lot, right?) to get a feel for where things are and then you can move faster on your next attempt.
If you've played as much as you say, then I'm not clear what you're missing. I just flattened Gateway after taking all enemy castles while only reloading once (I blindly attacked the sorceress castle, knowing that it was a bad idea, instead of building a Thieves Guild first to check for guards), and I might still have won if I'd played that out.
I've found HoMM5, where Ubisoft took over, to be FAR more gimmicky overall than any of the earlier campaigns I've played. I can't speak for 6+ because I haven't bothered to acquire them.
agof: well is there a way to get the dimension door? the spells are randomized?
Get a town with a level 4 mage guild to have a chance at it. Spells are randomized. If you go Warlock, then level 3 guild might give you storm, which also combos quite well with dragons.
Strong as it is, DD isn't the end-all be-all. You still need enough army/spell power to conquer the targets that DD helps you reach.